1. Field of the Invention
The present invention relates to a centrifugal blower, which has a centrifugal fan arranged in a scroll casing.
2. Description of Related Art
For example, Japanese Unexamined Patent Publication No. 10-196596 discloses a centrifugal blower shown in FIG. 10.
With reference to FIG. 10, the prior art centrifugal blower 101 has a centrifugal fan 110 arranged in a scroll casing 120. The centrifugal fan 110 includes a plurality of blades 111 arranged along a circle and is rotated by a motor (not shown). Upon rotation of the motor, the centrifugal fan 110 axially draws air through an air inlet 124 of the scroll casing 120 located at one axial end (axial end located on the back side of the sheet of FIG. 10) of the centrifugal fan 110 and then radially outwardly blows the drawn air. The scroll casing 120 forms a scroll air passage 126 around the centrifugal fan 110. The scroll air passage 126 extends from a nose 127 of the scroll casing 120, which serves as an initial point of the scroll air passage 126. A protrusion 123a is formed at the air inlet 124 to partially cover the air inlet 124.
At the nose 127, which serves as the initial point of the scroll air passage 126, an inner cylindrical surface 121 of an outer peripheral wall (a cylindrical wall that extends in the axial direction of the centrifugal fan 110) of the scroll casing 120 is in close proximity to a radially outer end of a corresponding one of the blades 111 of the centrifugal fan 110. Thus, a relatively large pressure is developed in that region. When the pressure in this region is increased, reverse airflow, which flows toward the air inlet 124, tends to occur. The protrusion 123a is provided to restrain escape of the reverse airflow through the air inlet 124.
In recent years, considerable efforts have been devoted to reducing a size of such a centrifugal blower, and it has been found that the reduction of the size of the centrifugal blower generally causes an increase in annoying siren-like noises in the centrifugal blower although the reverse airflow does not substantially escape through the air inlet. The siren-like noises are generated when the air radially outwardly discharged from the centrifugal fan impinges against the opposed inner cylindrical surface of the outer peripheral wall of the scroll casing.
FIG. 11 schematically shows the region around the nose 127 of the centrifugal blower 101. When the centrifugal fan 110 is rotated by the motor (not shown), the air drawn through the air inlet 124 is radially outwardly blown by the corresponding blade 111. Each time the respective blade 111 passes the nose 127, the air blown by the blade 111 impinges against the inner cylindrical surface 121 of the scroll casing 120 and generates the siren-like noises.
Lately, throughout extensive efforts, the inventors of the present invention have found that velocities of airflow components of the airflow, which is radially outwardly blown by the corresponding blade 11, differ from one another along a direction parallel to the rotational axis of the centrifugal fan 110, as indicated by blank arrows in FIG. 11.
When the air inlet 124 is arranged adjacent to the lower axial end of the centrifugal fan 110, as shown in FIG. 11, the velocity of the upper airflow component, which is located at the upper side of the centrifugal fan 110, is generally greater than the velocity of the lower airflow component, which is located at the lower side of the centrifugal fan 110, as shown in FIG. 11. Time of impingement of the upper airflow component against the inner cylindrical surface 121 of the scroll casing 120 is slightly faster than time of impingement of the lower airflow component against the inner cylindrical surface 121 of the scroll casing 120. However, a time difference between the time of impingement of the upper airflow component against the inner cylindrical surface 121 and the time of impingement of the lower airflow component against the inner cylindrical surface 121 is relatively small and insignificant. Furthermore, in a case of a small fan, particularly one that has a narrow nose gap, the velocity of the airflow at the time of impingement against the inner cylindrical surface is relatively high, causing generation of relatively large noises.
The noises, which are generated at the time of impingement of the airflow against the inner cylindrical surface 121, occur every time the respective blade 111 passes the nose 127, resulting in siren-like noises.